Over the years, many processes and many different types of apparatus have been developed for the embossing of synthetic thermoplastic sheet material. When continuous embossing operations are employed, generally embossing rolls are engraved with the desired embossing pattern and the sheet material is passed through a nip defined by the embossing roll and a cooperating roll. In some instances, embossing belts formed with an embossing pattern have been suggested and in other instances it has been suggested that the sheet material be step-fed through an embossing press with the material elevated to embossing temperature and then cooled in the press while clamping pressure is maintained
One of the problems inherent in embossing sheet material is to ensure that air or other gases do not become trapped in the cavities of the embossing mold so as to affect the quality of the embossed formations produced therein. Another problem inherent in embossing operations is that of avoiding deleterious effects upon the embossing mold because of the different coefficients of expansion of the metal mold and the plastic sheet material as the embossed sheet material begins to cool in place within the mold while still adhering to the embossing surface thereof. However, if the sheet material is not adequately held in precise contact with the embossing surface of the mold as it cools, there is a tendency for the embossed formations to distort or imperfectly form, and premature stripping of the embossed material from the molds may also adversely affect the quality of the formations.
These problems become of particular significance when microprism type retroreflective sheeting is sought to be produced in a continuous manner. Such microprism sheeting utilizing a highly controlled cube corner formation has heretofore been made by a molding or casting process as described and claimed in Rowland U.S. Pat. Nos. 3,684,348 granted Aug. 15, 1972 and 3,810,804 granted May 14, 1974. Variations of this technology are described and claimed in Rowland U.S. Pat. Nos. 3,689,346 granted Sept. 5, 1972 and 3,935,359 granted Jan. 27, 1976.
It has been previously proposed to emboss cube corner sheeting on a continuous basis as indicated in Jungersen U.S. Pat. Nos. 2,310,790 granted Feb. 9, 1943; 2,380,447, granted July 31, 1945; and 2,481,757 granted Sept. 13, 1949. However, the process and apparatus described therein have not proven satisfactory for generating microprism reflective sheeting.
Illustrative of other processes for molding or casting cube corner reflectors are Wilson U.S. Pat. No. 2,538,638 granted Jan. 16, 1951; Schultz U.S. Pat. No. 3,417,959 granted Dec. 24, 1968; and Swarovski Danish Pat. No. 60,837 published Apr. 19, 1943.
Such cube corner reflectors have been extensively molded from glass and from acrylic resins and are widely employed as safety devices on bicycles, automobiles and other vehicles, as well as for various other retroreflective safety purposes. Although Jungersen suggests that the cube corner formation might be of small dimension, efforts to fabricate microprism formations by conventional embossing techniques have been generally unsatisfactory because it is important that the size and angle of the prism be controlled to a very close tolerance since even a minute deviation will produce a substantial deviation in the capability of the material to retroreflect light rays impinging thereon.
Although conceivably conventional molding or embossing techniques could be employed wherein the sheet material were heated to a temperature sufficient to cause the plastic material to flow into the mold and were held under pressure for a period of time sufficient to permit escape of any air or other gases that might interfere with formation of the embossment and then to permit both mold and sheet material to cool to a temperature wherein the embossed sheet material might be stripped therefrom without injuring the embossments, generally such apparatus and procedures would not adapt themselves to formation of continuous sheeting except by a step process operated at relatively slow speeds.
It is an object of the present invention to provide a novel embossing method which is operable at relatively high speeds to semicontinuously emboss synthetic plastic sheet material with precisely formed embossments along the length thereof in a repeating pattern with substantially no deviation between the embossments of different repeats of the pattern within the series.
It is also an object to provide such a method which is relatively foolproof in operation and readily adaptable to producing different embossing formations.
Another object is to provide a method for producing precisely formed embossments upon synthetic plastic sheet material on a semicontinuous basis at a reasonably rapid rate.
Still another object is to provide such a method which enables close control of the formation of the embossments and substantial elimination of any deviation between the embossed formations of different repeats of the pattern in the series along the length thereof.
A further object is to provide synthetic thermoplastic sheet material in continuous roll form having precisely formed embossments along the entire length thereof with substantially no deviation between the embossments in repeats of the patterns along the length thereof.